The invention relates to an acid status indicator for a battery, in particular for a lead-acid rechargeable battery, having a sight glass and a holder for holding the sight glass and for attaching the acid status indicator to a housing of the battery. According to a second aspect, the invention relates to a battery. According to a third aspect, the invention relates to a method for the production of an acid status indicator and of a battery.
By way of example, acid status indicators are used in lead-acid rechargeable batteries, where they are used to indicate the state of charge. State of charge indicators such as these are based on the effect that the density of an electrolyte in the battery varies as a function of the state of charge of the battery. For this purpose, acid status indicators comprise buoyant bodies whose buoyancy depends on the density of the electrolyte. An indicator apparatus is arranged in the sight glass, determines the buoyancy of the buoyant body, and indicates the state of charge of the battery on this basis. A holder is provided in order to connect the sight glass to a housing in which the other components of the battery are arranged.
DE 298 21 994 U1 describes an acid status indicator in which the sight glass and the holder form an integral component. This has the disadvantage that a plastic which is particularly highly suitable for the sight glass is generally not very suitable as the material for the holder.
In the case of the acid status indicator that is known from DE 298 08 883 U1, the sight glass is welded to the holder. This has the disadvantage that the welding means an additional process step during production, which increases the manufacturing effort.
EP 1 432 061 A1 discloses an acid status indicator in which a sight glass is attached via a holder to a housing of the battery. In order to ensure a good sealing effect between the sight glass and the holder, soft-component seals are provided between the sight glass and the holder. EP 1 633 008 A1 describes a diagnostic probe for a starter battery, in which a sight glass is arranged in a holder on which a seal with a sealing lip, which points radially outwards, is arranged. The seals are disadvantageously subject to aging. Furthermore, the provision of a seal increases the manufacturing effort.
Other acid status indicators are produced by the sight glass being forced under pressure into the holder, thus resulting in an interference fit between the two. During the course of manufacturing the battery, the holder and the sight glass connected to it are then firmly connected to the housing of the battery. It has been found that acid status indicators such as these have the disadvantage that, even when an interference fit is provided, it is not possible to guarantee that the connection between the sight glass and the holder will be acid-tight and gas-tight. For this reason, the gas-tightness and acid-tightness of the acid status indicator must be checked after manufacture of the battery, and this is complex and expensive.
A further disadvantage is that a connection which is initially sealed between the sight glass and the holder can also become leaky when the conditions are not good.
DE 40 07 905 A1 discloses an indicating apparatus for a battery measurement sensor, in which a measurement sensor plate is held in an insert cap, with the measurement sensor plate having a 90° bend in the insert cap, and being held insert-molded in the insert cap.
DE 20 2005 019 009 U1 discloses an insulated battery pole for a lead-acid battery, in which the battery pole is produced by insert-molding of the electrode.
DE 202 20 044 U1 discloses an electrolyte circulating device for a lead-acid rechargeable battery, which has an air guide tube which is surrounded by an elastic sealing sleeve produced by insert molding. The sealing sleeve makes it possible to arrange the air guide tube, forming a seal, in a cylindrical piece, on the inside, of a retaining opening.
DE 78 22 820 U1 discloses a pole bushing for rechargeable batteries. In order to produce a seal with respect to the ingress of electrolyte, the electrode which forms the cell pole is insert-molded with a thermoplastic body.
The invention is therefore based on the object of specifying an acid status indicator which has a gas-tight and acid-tight connection, which can be adjusted in a manner which is reliable from the process point of view, between the sight glass and the holder.
The invention solves the problem by an acid status indicator having the features of claim 1. According to a second aspect, the invention solves the problem by a method for production of an acid status indicator having the features of claim 12.
The invention has the advantage that, as a result of the specific way in which the holder is produced, in a manner which is reliable from the process point of view, this results in a gas-tight and acid-tight connection between the sight glass and the holder. This avoids complex rechecking. Furthermore, it has been found that an acid status indicator produced according to the invention also does not lose its gas-tightness and acid-tightness during use.
Furthermore, the simpler manufacture is advantageous since this avoids the sight glass from being inserted under pressure into the holder. In the conventional manufacturing process, the sight glass and the holder are connected to one another in the cold state. To do this, the sight glass is pushed into the holder under pressure. In order to ensure that there is a contact-pressure force for the interference fit between the two after being pushed in, the sight glass must be elastically compressed and the holder expanded during insertion. Both the expansion and the compression are, however, subject to limits resulting from the strength of the plastics used. The sight glass can thus be inserted only with slight pressure, as a result of which only a relatively small contact-pressure force can be maintained between the sight glass and the holder.
If, according to the invention, the holder is produced by insert molding, then the plasticized plastic is located around the sight glass. On cooling, the plastic shrinks and exerts a strong contact-pressure force on the sight glass. Since the holder is produced by insert molding of the sight glass, this therefore results in a holder which is shrunk onto the sight glass, resting completely on the sight glass. This shrunk-on holder forms a particularly strong, gas-tight and acid-tight interference fit with the sight glass, in which an acid status indicator according to the invention differs from acid status indicators according to the prior art.
Plastics can therefore be used both for the sight glass and for the holder which, because of their brittleness, that is to say because of their small amount of strain to fracture, could otherwise not be used.
In the present description, the expression a sight glass means in particular a component which is used to indicate an acid status, in particular an acid density of the battery. The sight glass is preferably transparent or opaque. When the sight glass enters the acid, it thus appears dark at an end on the housing side, otherwise light. In particular, the sight glass makes it possible to see one or more balls, which are arranged in a ball cage, in order to indicate the acid density.
According to the invention, the sight glass and the holder are produced by two-component injection molding. In particular, this means that the sight glass and the holder are produced in a single two-component injection-molding method step, and in a single injection-molding machine.
In one preferred embodiment, the sight glass is composed of a first plastic and the holder is composed of a second plastic, with the first plastic shrinking less than the second plastic when it cools down. The sight glass and the holder are therefore braced with respect to one another when the plastics cool down, thus advantageously achieving a particularly good sealing effect between the sight glass and the holder. In this case, the shrinkage of the second plastic is preferably at least twice as great as that of the first plastic. The expression shrinkage means in particular the relative decrease in length when a plastic cools down from its plasticizing temperature to room temperature.
In one preferred embodiment, the sight glass comprises styrene/acrylonitrile or is composed of styrene/acrylonitrile.
In one preferred embodiment, the holder comprises polypropylene, and in particular the holder is composed of polypropylene. It is particularly advantageous for the sight glass to be produced from styrene/acrylonitrile and for the holder to be produced from polypropylene using a two-component injection-molding method, since polypropylene shrinks to a far greater extent than styrene/acrylonitrile, thus resulting in a gas-tight and acid-tight acid status indicator which is reliable from the process point of view, for the reasons mentioned above.
In one preferred embodiment, the sight glass has a bead which is surrounded by the holder in an interlocking manner. In this case, it is advantageous for the sight glass to be cylindrical in places and for the bead to be completely circumferential around a cylindrical section such as this. The sight glass is held particularly firmly in the holder by the interlock. Alternatively or additively, the sight glass has a projection which is surrounded by the holder in an interlocking manner. Both the bead and the projection may be completely or only partially circumferential.
The sight glass furthermore preferably has a stepped end in order to indicate of different acid states or levels.
The invention will be explained in more detail in the following text with reference to the drawing, which illustrates a detail of a housing of a battery according to the invention, which is connected to an acid status indicator according to the invention.